The present invention relates generally to the segment of the field of gas analysis instrumentation that involves the use of solid electrolyte cells to measure gaseous species. More specifically, the invention relates to a novel diagnostic method and apparatus for use with a heated solid electrolyte cell to assess the condition of such cell. As used herein, the term solid electrolyte cell means a quantity of the solid electrolyte, for example zirconia-yttria ((ZrO.sub.2).sub.(1-x) (Y.sub.2 O.sub.3).sub.x), and porous electrical contacts or electrodes connected thereto, usually made of platinum or other suitable materials.
Gas analyzers having a probe with a solid electrolyte cell to measure gaseous oxygen are well known. See, for example, U.S. Pat. Nos. 3,400,054 and 3,928,161, incorporated herein by reference. Another example is the World Class 3000 Oxygen Analyzer sold by Rosemount Analytical Inc. of Orrville, Ohio, available with replacement cells under part no. 4847B61G01/02/03. A common application for such analyzers is the measurement of gaseous oxygen in a flue or duct such as a smokestack. At a given elevated temperature, the solid electrolyte cell generates an EMF V.sub.cell as a function of the concentration of oxygen (or other gaseous specie) exposed to the cell. V.sub.cell can be approximated by the Nernst equation: EQU V.sub.cell =C+S.multidot.log{P(O.sub.2)/P(O).sub.REF }, (EQ. 1)
where C is a cell constant, S is a cell slope which is a function of cell temperature T, and P(O.sub.2) and P(O.sub.2).sub.REF are the oxygen partial pressure at a measurement and reference end, respectively, of the solid electrolyte cell. Actual solid electrolyte cells deviate from Equation 1 to some extent.
It is also known for solid electrolyte analyzers to include analog or digital electronic circuitry in a housing separate from the probe that measures V.sub.cell and provides an analyzer output indicative of the gaseous species concentration. The probe is configured with a heater and thermocouple, controlled by circuitry in the separate housing, to maintain the solid electrolyte cell at a constant elevated temperature (e.g., T.apprxeq.750.degree. C.).
It is also known to provide a pneumatic tube in a gas analyzer probe to supply reference gas with a known oxygen content P(O.sub.2).sub.REF to a reference side of the cell, and to provide another pneumatic tube that can when desired supply a calibration gas having an oxygen content P(O.sub.2).sub.CAL) to a measuring side of the cell. Usually the pneumatic tube supplying the calibration gas is closed and the measuring side of the cell contacts the gas of interest that is to be measured for oxygen content.
It is further known that solid electrolyte cells can degrade over their useful life, experiencing an increase in cell resistance and also experiencing a decrease in V.sub.cell.
An object of the invention is to provide an in situ indication of the condition of the solid electrolyte cell, without having to supply specialized calibration gas to the analyzer or otherwise interrupt the operation of the analyzer. Cell condition information can include time remaining to calibration, an out-of-calibration indication, time remaining to replacement, and a need-to-replace indication.
Other objects of the invention will become apparent from the detailed description of the invention and the accompanying drawings.